Arbuscular mycorrhizal fungi (AMF) strongly influence the terrestrial carbon cycle: they mediate primary productivity via their role in plant nutrient acquisition, and they represent an important belowground C sink. Conceptual models predict that ecological relationships between plants and AMF are mediated by resource stoichiometry, as the plant-fungal mutualism should be strongest when plants are limited by P. However, this prediction has not been evaluated in tropical dry forests, where pronounced intra-annual variation in rainfall generates strong seasonal variation in soil nutrient availability. Working in regenerating seasonal forests in northwest Costa Rica, we quantified fine root production and colonization by AMF along a steep natural gradient of soil P availability. We also examined shifts in the structure of AM fungal communities along the gradient, linking compositional variation to edaphic properties and aboveground plant communities. Finally, we examined changes in root colonization by AMF in response to a full-factorial, in situmanipulation of nitrogen and phosphorus availability.
Root colonization by AMF did not vary over a 50-fold gradient of total soil P, although fine root biomass tended to be greater in low-fertility soils. However, in the fertilization experiment, addition of N and P together reduced root colonization two-fold. Along the natural P gradient, the composition of AM fungal communities was not affected by the availability of any soil nutrient, but was significantly associated with plant community structure. Taken together, our results suggest that nutrient availability does influence allocation to arbuscular mycorrhizal fungi, but that the relative abundance of AMF taxa is more sensitive to functional diversity within plant communities. Tree species vary in their responsiveness to and dependence on AMF, complicating efforts to predict allocation to mycorrhizal fungi based on soil resource stoichiometry alone.